Ion implantation is a standard technique for introducing conductivity-altering impurities into a workpiece. A desired impurity material is ionized in an ion source, the ions are accelerated to form an ion beam of prescribed energy, and the ion beam is directed at the surface of the workpiece. The energetic ions in the beam penetrate into the bulk of the workpiece material and are embedded into the crystalline lattice of the workpiece material to form a region of desired conductivity.
Solar cells are one example of a device that uses silicon workpieces. Any reduced cost to the manufacture or production of high-performance solar cells or any efficiency improvement to high-performance solar cells would have a positive impact on the implementation of solar cells worldwide. This will enable the wider availability of this clean energy technology.
Solar cells are typically manufactured using the same processes used for other semiconductor devices, often using silicon as the substrate material. A semiconductor solar cell is a simple device having an in-built electric field that separates the charge carriers generated through the absorption of photons in the semiconductor material. This electric field is typically created through the formation of a p-n junction (diode) which is created by differential doping of the semiconductor material. Doping a part of the semiconductor substrate (e.g. surface region) with impurities of opposite polarity forms a p-n junction that may be used as a photovoltaic device converting light into electricity.
FIG. 1 shows a cross section of a representative solar cell 100, where the p-n junction 120 is located away from the illuminated surface. Photons 10 enter the solar cell 100 through the top (or illuminated) surface, as signified by the arrows. These photons pass through an anti-reflective coating 104, designed to maximize the number of photons that penetrate the substrate 100 and minimize those that are reflected away from the substrate. The ARC 104 may be comprised of an SiNX layer. Beneath the ARC 104 may be a passivation layer 103, which may be composed of silicon dioxide. Of course, other dielectrics may be used. On the back side of the solar cell 100 are an aluminum emitter region 106 and an aluminum layer 107. Such a design may be referred to as an Al back emitter cell in one instance.
Internally, the solar cell 100 is formed so as to have a p-n junction 120. This junction is shown as being substantially parallel to the bottom surface of the solar cell 100, although there are other implementations where the junction may not be parallel to the surface. In some embodiments, the solar cell 100 is fabricated using an n-type substrate 101. The photons 10 enter the solar cell 100 through the n+ doped region, also known as the front surface field (FSF) 102. The photons with sufficient energy (above the bandgap of the semiconductor) are able to promote an electron within the semiconductor material's valence band to the conduction band. Associated with this free electron is a corresponding positively charged hole in the valence band. In order to generate a photocurrent that can drive an external load, these electron-hole (e-h) pairs need to be separated. This is done through the built-in electric field at the p-n junction 120. Thus, any e-h pairs that are generated in the depletion region of the p-n junction 120 get separated, as are any other minority carriers that diffuse to the depletion region of the device. Since a majority of the incident photons 10 are absorbed in near surface regions of the solar cell 100, the minority carriers generated in the emitter need to diffuse to the depletion region and get swept across to the other side.
Some photons 10 pass through the front surface field 102 and enter the p-type emitter 106. These photons 10 can then excite electrons within the p-type emitter 106, which are free to move into the front surface field 102. The associated holes remain in the emitter 106. As a result of the charge separation caused by the presence of this p-n junction 120, the extra carriers (electrons and holes) generated by the photons 10 can then be used to drive an external load to complete the circuit.
By externally connecting the base through the front surface field 102 to the emitter 106 through an external load, it is possible to conduct current and therefore provide power. To achieve this, contacts 105, typically metallic and in some embodiments silver, are placed on the outer surface of the front surface field 102.
FIG. 2 illustrates a second embodiment of a solar cell 200. In this embodiment, the p-n junction is not created within the substrate 201. Rather, the substrate 201, typically an n-type silicon substrate, is isolated from the back surface by thin oxide tunnel layer 202. The tunnel oxide layer is sufficiently thin to allow electrons to tunnel through the layer 202, such as between 1 and 4 nanometers. This layer 202 may be silicon dioxide or another suitable dielectric material. On the opposite side of the tunnel oxide layer is a polysilicon layer 203. This layer has n-type regions 204a and p-type regions 204b, located adjacent to each other. Where these regions meet, a p-n junction 210 is formed. Contacts 25 are then applied to the n-type regions 204a and p-type regions 204b. In some embodiment, a second tunnel oxide layer 206 exists on the front surface of the substrate 201. In this embodiment, an n-type polysilicon layer 207 may be applied to the second tunnel oxide layer 206.
This embodiment of a solar cell has several advantages. First, its efficiency may be greater than the traditional solar cell of FIG. 1. This may be due to reduced recombination of carriers within the substrate.
However, the production of these polysilicon solar cells is time consuming and costly, requiring many process steps. Therefore, an improved method of manufacturing polysilicon solar cells is needed.